1. Field of the Invention
The invention concerns an apparatus and method for the separation in a fluidized bed of two mixed solid phases, one of which is formed by fluidized powdery materials while the other is made up of materials which are not fluidizable under the conditions of fluidization of the former materials.
The term "fluidizable materials" is used to denote all materials which are well known to the man skilled in the art and which occur in a powdery form and with a granulometry and cohesion such that the speed of flow of the injected air therethrough causes, at a low rate, decohesion of the particles from each other and a reduction in the internal frictional forces. Such materials are, for example, alumina which is intended for igneous electrolysis, cements, plasters, lime (either quicklime or slaked lime), fly ash, calcium fluoride, additive fillers for rubber; starches, catalysts, carbon dust, sodium sulphate, phosphates, pyrophosphates, plastics materials in the form of powder, foodstuff products such as powdered milk, flour, etc.
2. Related Art
It is well known that many methods have been the subject of investigation and development for fluidized bed transportation of powdery materials from a storage zone to at least one consumption zone to be supplied with such materials, with the zones being disposed at a distance from each other, such as for example a bag filling machine, a machine for putting material into containers, or a production assembly such as an extrusion press.
One problem involving the feed of alumina to ingneous electrolysis cells for producing aluminum is that of transporting a powdery material, the alumina, over a long distance. Stored in a silo of very large capacity, such material is intended for feeding processing workshops which are several hundreds of meters distant from the silo; a known solution to this problem has been the use of movable containers or a high pressure pneumatic conveyor system or mechanical transport arrangements.
Apparatuses for transporting alumina by a fluidized bed process have also been proposed. One of those apparatuses, which is intended for feeding alumina to electrolysis cells at multiple points, is described for example in U.S. Pat. No. 4,016,053. That apparatus which is proposed for conveying alumina from a storage zone to a consumption zone has a primary fluidized bed conveyor provided with means for feeding and discharging the gas used for permanently fluidizing the alumina and maintaining the primary conveyor substantially full of fluidized materials. A plurality of secondary fluidized bed conveyors are provided with the same means for feeding and discharging the fluidization gas, for receiving and transporting the powdery materials which come from the primary conveyor by maintaining them in the same state of permanent fluidization as in the primary conveyor, and apparatuses are provided for the discontinuous feed of powdery materials to each electrolysis tank.
Any apparatus for conveying a powdery material such as alumina in a fluidized bed mode operates satisfactorily as long as the material is homogenous, that is to say, as long as the powdery material to be transported constitutes a single fluidizable phase.
However, once the materials to be transported in the fluidized bed conveyors form two solid phases in a mixture with each other, one of which settles under the conditions of fluidization of the other phase, the fluidized bed conveyors suffer from serious disturbances which interfere with operation of the system and which can result in the flow of fluidized materials becoming blocked, since the materials which settle on the fluidization wall give rise to the formation of preferential gas flow paths. For that reason, transportation of alumina in fluidized bed conveyors may be adversely affected by the presence of another solid phase which settles under the conditions of fluidization of the alumina.
The problem arises in particular when recycling alumina in a system for feeding electrolysis tanks for producing aluminum. Alumina, because of its adsorption properties, is used for connecting the fluorine-bearing effluents emitted by the tanks in the operation thereof. That alumina which is charged with the collected effluent products tends to form compact agglomerates which are referred to by a term in the art as "scales" and which interfere with operation of the fluidized bed feed arrangements.